Process for the preparation of high alumina cement

ABSTRACT

High alumina cement is produced in a submerged combustion melter, cooled and ground.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national counterpart application ofInternational Application Serial No. PCT/EP2016/051731, filed Jan. 27,2016, under 35 U.S.C. § 371, which claims priority to GB Application.Serial No. 1501306.3, filed Jan. 27, 2015, the disclosures of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an improved process for the preparationof high alumina cement.

BACKGROUND OF THE INVENTION

High alumina cement, also called aluminous cement or calcium aluminatecement, is produced by fusing a mixture of limestone and bauxite at hightemperatures comprised between 1400 and 1600° C., hence obtaining a meltwhich after cooling is ground to fine cement. The fusion may beperformed in shaft furnaces, like blast furnaces, or in rotary kilns.

In typical formulations for the preparation of high alumina cement, thecontent of SiO₂ may vary from 0.4 to 10.0% by weight, Al2O3 may varyfrom 25 to 85% by weight and CaO from 15 to 50% by weight. High aluminacement may be used as hydraulic binder for the preparation of concreteintended for construction purposes or in the manufacture of refractoryelements. It may also be mixed with other cements for the preparation ofcement blends showing specific properties. High alumina cement is knownfor its rapid strength development.

The production of high alumina cement requires high energy inputs, andthere is an ever increasing need for improvement of the energyefficiency of the manufacturing process.

Moreover, because of the highly corrosive nature of the raw materialsand melt to be treated, the refractory lining of the furnaces in whichthe high alumina cement is treated needs to be repaired or replacedafter relatively short time periods. There is hence a need to find a wayto overcome that technical problem.

SUMMARY OF THE INVENTION

It has been found that high alumina cement may advantageously beprepared in a submerged combustion melter. The invention process hencecomprises introducing solid batch material for preparation of highalumina cement into a melter, melting the solid batch material in themelter by submerged combustion, discharging a liquid melt, cooling saiddischarged liquid melt to obtain solidified melt and grinding thesolidified melt to appropriate grain size. The grinding step is known tothe person skilled in the art and may be adapted to product qualitydemand and requirements of the market place.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are schematic representations of a toroidal flow patternin a submerged combustion melter;

FIG. 2 shows a vertical section through a submerged combustion melter;

FIG. 3 is a schematic representation of a burnout layer for a melter ofFIG. 2; and

FIG. 4 schematically shows a production line according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The preparation of the high alumina cement may be carried out using amethod and/or melter disclosed in any of WO 2015/014919, WO 2015/014920or WO 2015/014921, each of which is hereby incorporated by reference.

Submerged combustion melters are known. These melters are characterizedby the fact that they have one or more burner nozzles arranged below thesurface of the melt, in a lance, in the melter walls and/or melterbottom, preferably in the melter bottom, such that the burner flameand/or combustion products pass through the melt and transfer energydirectly to the melt.

Submerged combustion melters are known to generate high turbulence oragitation in the melt caused at least partially by the injection ofcombustion gas under high pressure into the melt and by the convectivemovements within the melt. The high turbulence ensures efficient mixingin the melt, and homogenizes the melt in terms of temperature profileand composition, leading to a high quality cement product. It alsofavors the absorption of raw material into the melt and improves heattransfer to fresh raw material. This reduces required residence time inthe melter prior to withdrawal for downstream treatment. It ispreferred, however, that the burners are controlled such that the meltvolume is increased by at least 8%, preferably at least 10%, morepreferably at least 15% or 20%, compared to the volume the melt wouldhave with no burners firing. It is understood that the gas injectionreduces the density of the melt, hence increases its volume, compared towhat it would be when no gas is being injected.

In connection with the above, the melt volume (no submerged burnersfiring) may be calculated as a function of the temperature and the rawmaterial batch composition. The level and hence volume of the agitatedmelt (submerged burners firing) may be measured with laser scanners orsimilar measuring devices that allow to measure and average melt levelover a given period of time.

While submerged combustion has a tendency to cause foam formation at thetop of the melt, that is over the melt level, it is preferable tooperate the submerged combustion melter without foam or at reduced foamlevel, as the foam level may be disadvantageous with respect to the heattransfer.

Furthermore, the melting chamber walls are preferably cooled; forexample, they may comprise double steel walls separated by circulatingcooling liquid, preferably water. Particularly in the case of acylindrical melting chamber, such assembly is relatively easy to buildand is capable of resisting high mechanical stresses. A cylindricalshape of the melter facilitates balance of stresses on the outside wall.As the walls are cooled, for example water cooled, melt preferablysolidifies and forms a protective layer on the inside of the melterwall. The melter assembly may not require any internal refractory liningand therefore needs less or less costly maintenance. In addition, themelt is not contaminated with undesirable components of refractorymaterial normally eroded from an internal refractory lining. Theinternal face of the melter wall may advantageously be equipped withtabs or pastilles or other small elements projecting towards the insideof the melter. These may help in constituting and fixing a layer ofsolidified melt on the internal melter wall generating a lining havingthermal resistance and reducing the transfer of heat to the coolingliquid in the double walls of the melter.

The melter may be equipped with heat recovery equipment. Hot fumes fromthe melter may be used to preheat raw material or the thermal energycontained in them may be recovered. Similarly, the thermal energycontained in the cooling liquid circulating between the two walls of themelter may also be recovered for heating or other purposes

Overall the energy efficiency of submerged combustion melters issignificantly improved compared to conventional shaft melters or rotarykilns.

The raw materials may be loaded through an opening in the melter wall,above the melt surface. Said opening may be opened and closed, forexample by a piston, to minimize escape of heat and fumes. Raw materialmay be prepared and loaded into an intermediate chute and subsequentlyfall into the melter, in an opposite direction to escaping fumes, ontothe melt surface. This countercurrent flow may advantageously preheatthe raw materials. In the alternative, the raw materials may be chargedbelow the level of the melt, by way of a screw feeder or a hydraulicfeeder.

Melt may be withdrawn continuously or batch wise from the melter. Whereraw material is loaded close to the melter wall, the melt outlet ispreferably arranged opposite the material inlet. In a preferredembodiment of the invention, the melt is withdrawn through a dischargeopening controlled by, for example, a ceramic piston. The piston mayopen or close a sliding door covering or uncovering the dischargeopening.

The submerged burners preferably inject high pressure jets of combustionproducts into the melt that is sufficient to overcome the liquidpressure and to create forced upward travel of the flame and combustionproducts. The speed of the combustion and/or combustible gases, notablyat the exit from the burner nozzle(s), may be 60 m/s, 100 m/s or 120 m/sand/or 350 m/s, 330 m/s, 300 or 200 m/s. Preferably the speed of thecombustion gases is in the range of about 60 to 300 m/s, preferably 100to 200, more preferably 110 to 160 m/s.

The temperature of the melt may advantageously be between 1400° C. and1600° C.; it may be at least 1450° C. or 1480° C. and/or no more than1600° C. or 1550° C. or 1520° C.

According to a preferred embodiment, the submerged combustion isperformed such that a substantially toroidal melt flow pattern isgenerated in the melt, having a substantially vertical central axis ofrevolution, comprising major centrally inwardly convergent flows at themelt surface; the melt moves downwardly at proximity of the verticalcentral axis of revolution and is recirculated in an ascending movementback to the melt surface, thus defining an substantially toroidal flowpattern.

The generation of such a toroidal flow pattern ensures highly efficientmixing of the melt and absorption of raw material into the melt, andhomogenizes the melt in terms of temperature profile and composition,thus leading to high quality final product.

Advantageously, the melting step comprises melting the solid batchmaterial, in a submerged combustion melter by subjecting the melt to aflow pattern which when simulated by computational fluid dynamicanalysis shows a substantially toroidal melt flow pattern in the melt,comprising major centrally inwardly convergent flow vectors at the meltsurface, with the central axis of revolution of the toroid beingsubstantially vertical.

At the vertical axis of revolution of said toroidal flow pattern, theflow vectors have a downward component reflecting significant downwardmovement of the melt in proximity of said axis. Towards the melterbottom, the flow vectors change orientation showing outward and thenupward components.

Preferably the fluid dynamics model is code ANSYS R14.5, taking intoconsideration the multi-phase flow field ranging from solid batchmaterial to liquid melt and gas generated in the course of theconversion, and the batch-to-melt conversion.

A toroidal melt flow pattern may be obtained using submerged combustionburners arranged at the melter bottom in a substantially annular burnerzone imparting a substantially vertically upward directed speedcomponent to the combustion gases. Advantageously, the burners arearranged with a distance between adjacent burners of about 250-1250 mm,advantageously 500-900 mm, preferably about 600-800, even morepreferably about 650-750 mm. It is preferred that adjacent flames do notmerge.

Each burner axis and/or a speed vector of the melt moving upwards overor adjacent to the submerged burners may be slightly inclined from thevertical, for example by an angle which is ≥1°, ≥2°, ≥3° or and/or whichis ≤30°, preferably ≤15°, more preferably ≤10°, notably towards thecenter of the melter. Such an arrangement may improve the flow anddirects melt flow away from the outlet opening and/or towards a centerof the melter thus favoring a toroidal flow and incorporation of rawmaterial in to the melt.

According to a one embodiment, each central burner axis is inclined by aswirl angle with respect to a vertical plane passing through a centralvertical axis of melter and the burner center. The swirl angle may be≥1°, ≥2°, ≥3°, ≥5° and/or ≤30°, ≤20°, ≤15° or ≤10°. Preferably, theswirl angle of each burner is about the same. Arrangement of each burneraxis at a swirl angle imparts a slightly tangential speed component tothe upward blowing flames, thus imparting a swirling movement to themelt, in addition to the toroidal flow pattern.

The burner zone is defined as a substantially annular zone. Burnerarrangements, for example on an elliptical or ovoid line within therelevant zone are possible, but the burners are preferably arranged on asubstantially circular burner line.

Preferably, the flow pattern comprises an inwardly convergent flow atthe melt surface followed by a downwardly oriented flow in proximity ofthe central axis of revolution of the toroid. Said central axis ofrevolution advantageously corresponds to the vertical axis of symmetryof the melter. By axis of symmetry is meant the central axis of symmetryand, if the melter shows a transversal cross-section which does not haveany single defined axis of symmetry, then the axis of symmetry of thecircle in which the melter section is inscribed. The downwardly orientedflow is followed by an outwardly oriented flow at the bottom of themelter and a substantially annular upward flow at proximity of theburners, reflecting recirculation of melt toward the burner zone and inan ascending movement back to the melt surface, thus defining asubstantially toroidal flow pattern.

The inwardly convergent flow vectors at the melt surface advantageouslyshow a speed comprised between 0.1-3 m/s. The downward oriented speedvectors at proximity of the vertical central axis of revolution arepreferably of significant magnitude reflecting a relatively high speedof material flowing downwardly. The downward speed vectors may bebetween 0.1-3 m/s. The melt and/or the raw materials within the melter,at least at one portion of the melter and notably at the melt surface(particularly inwardly convergent flow vectors at the melt surface)and/or at or proximate a vertical central axis of revolution, may reacha speed which is ≥0.1 m/s, ≥0.2 m/s, ≥0.3 m/s or ≥0.5 m/s and/or whichis ≤2.5 m/s, ≤2 m/s, ≤1.8 m/s or ≤1.5 m/s.

The preferred toroidal flow pattern ensures highly efficient mixing andhomogenizes the melt in terms of temperature profile and composition. Italso favors the absorption of raw material into the melt and improvesheat transfer to fresh raw material. This reduces required residencetime in the melter prior to withdrawal, while avoiding or at leastreducing the risk of raw material short cutting the melt circulation.

In one preferred embodiment, the burners are arranged at a distance ofabout 250-750 mm from the side wall of said melting chamber; this favorsthe preferred flow described above and avoids flame attraction to themelting chamber side walls. Too small a distance between burners andside wall may damage or unnecessarily stress the side wall. While acertain melt flow between burner and wall may not be detrimental and mayeven be desirable, too large a distance will tend to generateundesirable melt flows and may create dead zones which mix less with themelt in the center of the melter and lead to reduced homogeneity of themelt.

The distance between submerged burners is advantageously chosen such asto provide the desired toroidal flow pattern within the melt but also toavoid that adjacent flames merge. While this phenomenon depends on manyparameters such as temperature and viscosity of the melt, pressure andother characteristics of the burners, it has been found advantageous toselect a burner circle diameter comprised between about 1200 and 2000mm. Depending on burner type, operating pressure and other parameters,too large a diameter will lead to diverging flames; too narrow adiameter will lead to merging flames.

Preferably at least 6 burners are provided, for example arranged on aburner circle line, more preferably 6 to 10 burners, even morepreferably 6 to 8 burners, depending on the melter dimensions, burnerdimensions, operating pressure and other design parameters.

Each burner or each of a plurality of a group of burners, for exampleopposed burners, may be individually controlled. Burners close to a rawmaterial discharge may be controlled at different, preferably higher gasspeeds and/or pressures than adjacent burners, thus allowing forimproved heat transfer to the fresh raw material that is being loadedinto the melter. Higher gas speeds may be required only temporarily,that is, in the case of batch wise loading of fresh raw material, justduring the time period required for absorption of the relevant load intothe melt contained in the melter. It may also be desirable to controlburners that are located close to a melt outlet at a lower gasspeed/pressure in order not to disturb the outlet of the melt.

The melting chamber is preferably substantially cylindrical in crosssection; nevertheless, it may have an elliptical cross section orpolygonal cross section showing more than 4 sides, preferably more than5 sides.

It has been found that the melt for the preparation of high aluminacement shows a tendency to crystallize rather quickly. It may thus bedesirable to discharge the melt quickly for downstream solidificationand grinding. Such discharge may preferably be carried out through anoutlet opening which may be opened and closed by a sliding doorcontrolled by a piston.

The composition of the melt produced may typically comprise:

Possible melt composition (% weight) SiO₂ 4.0 Al₂0₃ 39.4 CaO 38.4 Fe₂O₃(total iron) 16.4 MgO 1.0 Na2O 0.1 K2O 0.2 TiO2 1.9 other Rest to 100%

The discharged melt is allowed to cool at suitable temperature forstorage and/or grinding. Grinding may be operated in several stages asis known per se. A first grinding step may break the cooled solidifiedmelt particles down to a particle size suitable for supply into agrinder that will finally reduce the particle size such that 100%thereof pass a 90 μm screen in a dry circuit, possibly in severalstages. Equipment for carrying out said grinding operations are known inthe art.

An embodiment of a melter suitable for use in accordance with thepresent invention is described below, with reference to the appendeddrawings of which:

FIGS. 1a and 1b are schematic representations of a toroidal flow patternin a submerged combustion melter;

FIG. 2 shows a vertical section through a submerged combustion melter;

FIG. 3 is a schematic representation of a burner layout for a melter ofFIG. 2; and

FIG. 4 schematically shows a production line according to the invention.

With reference to the figures, a toroidal flow pattern is preferablyestablished in which melt follows an ascending direction close tosubmerged burners 21, 22, 23, 24, 25, 26 which are arranged on acircular burner line 27, flows inwardly towards the center of thecircular burner line at the melt surface, and flows downwards in theproximity of the said center. The toroidal flow generates agitation inthe melt, ensures good stirring of the melt, and absorption of rawmaterial into the melt. Furthermore, it has been determined that theflow as generated also reduces foam generation at the top of the melt;the gas or foam bubbles being entrained back into the melt, thusreducing its density.

The illustrated melter 1 comprises: a cylindrical melting chamber 3having an internal diameter of about 2.0 m which contains the melt; anupper chamber 5; and a chimney for evacuation of the fumes. The upperchamber 5 is equipped with baffles 7 that prevent any melt projectionsthrown from the surface 18 of the melt being entrained into the fumes. Araw material feeder 10 is arranged at the upper chamber 5 and isdesigned to load fresh raw material into the melter 1 at a point 11located above the melt surface 18 and close to the side wall of themelter. The feeder 10 comprises a horizontal feeding means, for examplea feed screw, which transports the raw material mix to a hopper fastenedto the melter, the bottom of which may be opened and closed by avertical piston. The bottom of the melting chamber comprises sixsubmerged burners 21, 22, 23, 24, 25, 26 arranged on a circular burnerline 27 concentric with the melter axis and having a diameter of about1.4 m. The melt may be withdrawn from the melting chamber 3 through acontrollable outlet opening 9 located in the melting chamber side wall,close to the melter bottom, substantially opposite the feeding device10. The melt withdrawn from the melter may then be allowed to cool andsubsequently ground as required.

The temperature within the melt may be between 1400° C. and 1600° C.,preferably 1450° C. and 1550° C., depending on the composition of themelt, desired viscosity and other parameters. Preferably, the melterwall is a double steel wall cooled by a cooling liquid, preferablywater. Cooling water connections provided at the external melter wallallow a flow sufficient to withdraw energy from the inside wall suchthat melt can solidify on the internal wall and the cooling liquid, herewater, does not boil. The internal melter wall is not lined with anyrefractory material.

The submerged burners 21,22,23,24,25,26 comprise concentric tube burnersoperated at gas flows of 100 to 200 m/s, preferably 110 to 160 m/s andgenerate combustion of fuel gas and oxygen containing gas within themelt. The combustion and combustion gases generate agitation within themelt before they escape into the upper chamber and then through thechimney. These hot gases may be used to preheat the raw material and/orthe fuel gas and/or oxidant gas (e.g. oxygen, industrial oxygen have anoxygen content 95% by weight or oxygen enriched air) used in theburners. The fumes are preferably filtered prior to release to theenvironment, optionally using dilution with ambient air to reduce theirtemperature prior to filtering.

With reference to FIG. 4, raw material from a raw material storage 30 ischarged into the furnace 1 as described above, and withdrawn thereof forcooling 32 and further downstream treatments known per se. Thedischarged melt is allowed to cool at a temperature suitable for furtherdownstream operation, including grinding 34 to appropriate grain sizeand/or storing 36,38. The grinding is advantageously effected in severalstages, including a first stage 39 that reduces the particle size of thesolidified melt to a size suitable for downstream fine grinding 40 whichin turn may be carried out in a manner known per se, in several stages,in order to reach a particle size as is common in the cementmanufacturing industry. Mostly the final grain size is a powdery grainsize. For example, it is such that 100% of the particles pass a 90 μmscreen in a dry circuit. The production line further comprises dryers asappropriate and as is known per se; these devices have not been shown inthe figures.

With respect to the exemplified melter, it has been found that theturbulent aerated melt showed almost no foam floating at the top of themelt, and it has been determined that the turbulent aerated melt showeda volume (averaged over a 1 minute time period) of 30-50% higher thanthat calculated on the basis of the raw material fed into the melter andmaintained at the same temperature. The volume was.

The high alumina cement obtained is of high quality. The above describedproduction process is less energy demanding then known processes,because of the choice of submerged combustion melters that allow forimproved energy transfer to the melt, shorter residence times and thusless heat loss, and because the high stirring leads to a more homogenousmelt at reduced melt viscosity, which in turn may allow for operation atreduced temperatures. Furthermore, submerged combustion mayadvantageously be performed in water-cooled melters which are easier andless costly to maintain and repair and which further allow for recyclingof the energy withdrawn from the cooling fluid.

What is claimed is:
 1. A process for the preparation of high aluminacement comprising: introducing a solid batch material for preparation ofhigh alumina cement into a melter comprising submerged burners andmelting chamber walls; melting the solid batch material in the melter bysubmerged combustion to form a liquid melt; withdrawing at least aportion of the liquid melt from the melter; cooling said dischargedliquid melt to obtain a solidified melt; and grinding the solidifiedmelt to appropriate grain size; wherein the melting chamber walls arecooled using double steel walls separated by circulating cooling liquidand are not covered by a refractory lining.
 2. The process of claim 1,wherein heat is recovered from the process.
 3. The process of claim 2,wherein the recovered heat is used to preheat the raw materials.
 4. Theprocess of claim 1, wherein at least a part of the liquid melt iswithdrawn continuously or batchwise from the melter.
 5. The process ofclaim 1, wherein the submerged burners are controlled such that the meltvolume is increased by at least 8% as compared to the volume the meltwould have with no burners firing.
 6. The process of claim 1, whereinthe submerged combustion is performed such that a toroidal melt flowpattern is generated in the melt, having a vertical central axis ofrevolution, comprising major centrally inwardly convergent flows at themelt surface; wherein the melt moves downwardly at proximity of thevertical central axis of revolution and is recirculated in an ascendingmovement back to the melt surface for defining a toroidal flow pattern.7. The process of claim 1, wherein the melting step comprises meltingthe solid batch material in a submerged combustion melter by subjectingthe melt to a flow pattern which, when simulated by computational fluiddynamic analysis, comprises a toroidal melt flow pattern in the melt,comprising major centrally inwardly convergent flow vectors at the meltsurface, and comprising a vertical central axis of revolution.
 8. Theprocess of claim 7, wherein towards the melter bottom, the flow vectorschange orientation, showing outward and then upward components.
 9. Theprocess of claim 1, wherein the process is performed using productionequipment comprising (i) a submerged combustion melter (1) comprisingmelting chamber (3) comprising melting chamber walls (19) and a meltingchamber bottom, submerged burners (21, 22, 23, 24, 25, 26), and equippedwith a raw material discharge (10) or feeder and melt outlet (9), (ii) amelt cooling station and (iii) a grinding station.
 10. The process ofclaim 9, wherein the melting chamber walls (19) are cooled using doublesteel walls separated by circulating cooling liquid, and are not coveredby a refractory lining.
 11. The process of claim 9, wherein thesubmerged combustion burners (21, 22, 23, 24, 25, 26) are arranged atthe melter bottom in an annular burner zone.
 12. The process of claim 9,wherein the submerged combustion burners (21, 22, 23, 24, 25, 26) arearranged with a distance between adjacent burners of about 250-1250 mm.13. The process of claim 9, wherein each burner axis and/or a speedvector of the melt moving upwards over or adjacent to the submergedburners (21, 22, 23, 24, 25, 26) is slightly inclined from the verticalat an angle of greater than or equal to 1° and/or less than or equal to30° towards the center of the melter.
 14. The process of claim 9,wherein each central burner axis is inclined by a swirl angle of greaterthan or equal to 1° and/or less than or equal to 30° with respect to avertical plane passing through a central vertical axis of the melter andthe burner center.
 15. The process of claim 1, wherein heat is recoveredfrom the cooling liquid.